Hank, had to do it once.
Planned FDP 15:20
Started with some delay at the first station.
Colleague who took the first rest came back after 4 hours saying that he couldn't sleep.
I was getting tired at the end of the first leg so we took the decision to have a layover in OVB.
True, it's not easy as you know it might cause delays and mess up your colleague's schedules.
On the other hand, it is your duty as a crew member to take those decisions.